BUG: LibXml2 is required for building.

Previous patch turned this off, but it is
required for building libarchive.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22009 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: added mode for enabling/disabling Fiber editing in TractographyDisplay module and vtkMRMLTractographyDisplayDisplayableManager

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22010 3bd1e089-480b-0410-8dfb-8563597acbee